Photothermographic materials are well known in the photographic art. Photothermographic materials are also known as heat developable photographic materials. The photothermographic materials after imagewise exposure are heated to moderately elevated temperatures to produce a developed image without the need for processing solutions or baths. The heat development provides a developed silver image.
An example of a known photothermographic silver halide material comprises (a) photosensitive silver halide, prepared either in situ or ex situ, (b) an image forming combination comprising (i) an organic heavy metal salt oxidizing agent, generally a silver salt of a long chain fatty acid, such as silver behenate or silver strearate, with (ii) a reducing agent for the organic heavy metal salt oxidizing agent, such as a phenolic reducing agent, and, (c), generally a binder, such as poly(vinyl butyral). Such a photothermographic material is described in, for example, Research Disclosure, Vol. 170, June, 1978, Item No. 17029 and U.S. Pat. No. 4,264,725. It has been desirable to have photosensitive silver halide grains prepared ex situ in such a photothermographic material because silver halide has high photosensitivity and due to the ease of control in preparation of silver halide based on conventional aqueous silver halide gelatino emulsion technology. It has also been desirable to provide increased development efficiency, increased photographic speed, increased maximum density and more neutral tone developed images without the need for further addenda in such photothermographic materials containing photosensitive silver halide prepared ex situ. Adding conventionally prepared cubic grain silver halide gelatino photographic emulsions has not provided an answer to these problems as illustrated in the following comparative examples. No answer to these problems has been clear from the photothermographic art.